1. Field of the Invention
The present invention relates to an image forming apparatus, an image forming method, and a process cartridge each of which allows for stably forming high-resolution images over a long period of time while preventing occurrences of abnormal images that would be caused by repetitive use in high-speed operation by using a latent electrostatic image bearing member (may be hereinafter referred to as “photoconductor” and “electrophotographic photoconductor”) which has a surface layer and a photosensitive layer each having high-abrasion resistance, high-durability and excellent electric properties.
2. Description of the Related Art
In recent years, technologies of information processing system machines using an electrophotographic method have been remarkably advanced. In particular, performance of laser printers and digital copiers of which information is converted into digital signals to record the information by means of laser has been significantly enhanced in terms of printing quality and reliability. These laser printers and digital copiers which are widely used further require higher-quality imaging, higher-recording speed as well as down-sizing. Further, recently, demands for full-color laser printers and full-color digital copiers allowing for printing full-color images have become bulged rapidly. When an image is printed in full-color, at least four color toner-images should be superimposed, and thus further importance is placed particularly on technologies allowing for higher-recording speed and down-sizing of apparatuses.
To achieve higher-recording speed and down-sizing of these apparatuses, the photosensitivity of electrophotographic photoconductors used for these apparatuses must be enhanced, and such electrophotographic photoconductors must be down-sized. In particular, in case of a tandem-type image forming apparatus, which is effectively used for achieving both imaging in full-color and high-speed operation, at least four photoconductors are incorporated into the image forming apparatus, and thus the request for down-sizing of such a photoconductor is significantly high.
With advancement in technologies of making photoconductors have smaller diameters, photoconductors cannot avoid being used under harsher environments. Therefore, with conventional photoconductors, the exchange rate thereof is drastically increased, and a further serious problem may be caused particularly in high-speed machines. Consequently, giving photoconductors used together with image forming apparatuses not only higher-photosensitivity but also a remarkable degree of high-durability is essential to achieve higher-speed recording and down-sizing of those apparatuses.
In image forming apparatus which are operated at high-speed, a method in which a toner image formed on a photoconductor is directly transferred onto a recording medium is often employed as a transferring unit. In the method, a recording medium is conveyed using a belt to make it contact with the photoconductor surface or is conveyed to a proximate position to the photoconductor surface, and the toner image is transferred from the photoconductor surface to the recording medium by applying a sufficient amount of bias to the recording medium from the back side thereof. An image forming apparatus configured to operate at high-speed must be operated at high-linear velocity because of its design of the machine, and to increase the transferring rate, the transfer bias should be substantially increased. As the result, hazard applied to corresponding regions the toner developed on a photoconductor and other regions of the photoconductor significantly influences image-formation even via a recording medium, thereby causing abnormal images. For example, when the polarity of a transfer bias is inverse to the charge polarity of the photoconductor and the photoconductor is charged to the extent of a polarity which is inverse to the charge polarity of the photoconductor, the electric potential cannot be cancelled by removal of electricity, and the previous history of the latent electrostatic image remains on the photoconductor surface to cause a residual negative image.
To give higher-photosensitivity to a photoconductor, which is required to give high-speed processing performance to image forming apparatus, a charge generating material having a large quantum efficiency is essential. In organic high-photosensitive photoconductors, titanyl phthalocyanine having at least a maximum diffraction peak (±0.2°) of Bragg angle 2θ in XRD (CuKα ray) (wavelength: 1.542 angstroms) at 27.2 is widely used.
To give higher-durability to a photoconductor, stability of quality of images should be enhanced, in particular, occurrences of background smear should be prevented. For the mechanism of occurrences of background smear, it is considered that such a phenomenon is attributable to the following, i.e., when a charge is applied to a photoconductor, the charge is induced to a conductive substrate, another charge having a polarity which is inverse to the polarity of the above-noted charge is leaked locally and infused to the photosensitive layer and further infused to the surface of the photoconductor, and then the sites are easily developed. As two major factors that affect quality of images in repetitive use of a photoconductor, static charge fatigue of the photoconductor and abrasion of the photoconductor surface are exemplified. With respect to the former, a fatigue of a photoconductor is worse due to repetitive charging and exposing of the photoconductor at the time of forming an image, and reductions in electric potential of charge caused by the fatigue or increases in electric potential caused by the fatigue result in degradation of quality of images. In particular, a reduction in electric potential of charge further increases influence of the leaked charge from the conductive substrate to cause background smear. With respect to the latter, the surface layer of the photoconductor is worn away due to friction with a cleaning member or the like, and then the film thickness of the photoconductor surface layer is reduced, consequently, degradation of quality of images is caused due to an increased charge intensity and increased scratches on the photoconductor surface, and the like. Particularly when the electric field is increased by a reduction in film thickness, occurrences of background smear increases conspicuously.
For the reason, a charge transporting layer or a protective layer to be formed at the outermost surface of a photoconductor has been designed to improve the abrasion resistance.
For technologies to improve abrasion resistance of a photosensitive layer, (i) a photoconductor using a curable binder for a crosslinked charge transporting layer (for example, see Japanese Patent Application Laid-Open (JP-A) No. 56-48637), (ii) a photoconductor using a polymer charge transporting material (for example, see Japanese Patent Application Laid-Open (JP-A) No. 64-1728), and (iii) a photoconductor of which an inorganic filler is dispersed in a crosslinked charge transporting layer (for example Japanese Patent Application Laid-Open (JP-A) No. 4-281461) are exemplified. Since changes in electric intensity with time can be reduced by enhancing abrasion resistance of a photoconductor, a particularly high-effect can be obtained for preventing occurrences of background smear.
However, among these photoconductors, (i) the photoconductor using a curable binder is insufficient in solubility with charge transporting materials, and thus a residual charge potential tends to be increased due to effect of impurities such as polymerization initiator and unreacted residues to thereby cause degradation in image density. Further, (ii) the photoconductor using a polymer charge transporting material makes it possible to improve abrasion resistance thereof to some extent, however, the photoconductor has not yet attained the level to satisfactorily satisfy durability required for organic photoconductors. In addition, since it is difficult to polymerize and refine a polymer charge transporting material and it is hard to obtain a highly pure polymer charge transporting material, the electric properties are rarely stabilized in the material. Further, a problem in production may be caused, for example, a coating solution using a polymer charge transporting material has high viscosity. (iii) the photoconductor in which an organic filler is dispersed in a crosslinked charge transporting layer can exert high-abrasion resistance as compared to a typical photoconductor in which a low-molecular weight charge transporting material is dispersed in an inactive polymer, however, residual electric potential tends to be easily increased due to charge traps residing on the inorganic filler surface to thereby cause degradation in image density. When the degree of convexoconcave or irregularity induced to the inorganic filler and a binder resin of the photoconductor surface is great, cleaning defects may occur to cause toner filming and image deletion. Thus, with the use of the photoconductors of (i), (ii), and (iii), there are still problems with residual electric potential and surface cleaning property, which may cause image defects, and the proposed photoconductors have not yet attained the level to satisfy the required durability.
Further, as a technology of using a curable resin, a photoconductor is known in which a hardened material of a polyfunctional acrylate monomer is contained to improve abrasion resistance and scratch resistance (see, Japanese Patent (JP-B) No. 3262488). However, the invention described that the hardened material of the polyfunctional acrylate monomer is contained in a protective layer formed on a photosensitive layer, however, only described that a charge transporting material may be contained in the protective layer and there is no further detailed description. In addition, when a low-molecular weight charge transporting material was simply contained in a crosslinked charge transporting layer, there was a difficulty with solubility with the hardened material, and insufficient solubility with the hardened material actually causes precipitation of the low-molecular weight charge transporting material and white turbidity phenomenon to cause increases in electric potential in exposed regions. The increases in electric potential result in not only a degraded image density but also a degraded mechanical strength. Specifically, in the photoconductor, a monomer is reacted in a condition where a polymer binder is contained, and therefore, a three-dimensional (3D) network is not sufficiently formed and the crosslink density is sparse, and thus the photoconductor has not yet attained the level of exerting remarkable abrasion resistance.
As an alternative technology for improving abrasion resistance of a photosensitive layer, a technology is known in which a charge transporting layer is formed by using a coating solution composed of a monomer having a carbon-carbon double bond, a charge transporting material having a carbon-carbon double bond, and a binder resin (for example, see Japanese Patent (JP-B) No. 3194392). The binder resin is considered to play a role in improving adhesiveness between a charge generating layer and a hardened charge transporting layer and further alleviating internal stress of a film when the film is thickened and hardened, and such binder resin is broadly classified into a binder resin having a carbon-carbon double bond and having reactivity to the charge transporting material, and a binder resin which does not have a carbon-carbon double bond nor reactivity. The photoconductor allows for achieving both abrasion resistance and excellent electric properties and draws attention, however, when a binder resin having no reactivity is used, the solubility between the binder resin and a hardened material prepared by reacting the monomer with the charge transporting material is poor, layer separation occurs in the crosslinked charge transporting layer, which may cause scratches of the photoconductor surface, and adhesion of external additives and paper powder in a toner. As described above, a three-dimensional (3D) network is not sufficiently formed and the crosslink density is sparse, and accordingly the photoconductor has not yet attained the level of exerting remarkable abrasion resistance. Monomers exemplified as the monomer to be used for the photoconductor are bifunctional monomers, and in view of the above, the photoconductor has not yet attained the level to satisfy required abrasion resistance. Even when a binder resin having reactivity is used, although the molecular mass of the hardened material is increased, the number of crosslinked molecules is also small, it is difficult to achieve both sufficient bonding amount and sufficient crosslink density of the charge transporting material, and it cannot be said that the photoconductor meets a satisfactory level of electric property and abrasion resistance.
To solve the above-noted problems, a propose is known in which a layer hardened by irradiating a radical polymerizable monomer having no charge transporting structure and a radical polymerizable monomer having a charge transporting structure with an optical energy by means of an optical energy irradiating unit is provided as a protective layer (for example, see Japanese Patent Application Laid-Open (JP-A) No. 2004-302451). As the photoconductor does not contain a binder resin in the protective layer, a three-dimensional (3D) network is sufficiently formed and the crosslink density is substantially increased. For the reason, the photoconductor enables to exert abrasion resistance remarkably. Further, it is possible to achieve both satisfactory abrasion resistance and satisfactory electric properties because the charge transporting material is crosslinked.
Furthermore, a photoconductor is proposed which achieves continuation of low-surface energy of a photosensitive layer and improvement in transferring property and surface cleaning property over a long period of time by forming a surface layer which is hardened by irradiating a radical polymerizable monomer having no charge transporting structure, a radical polymerizable compound having a charge transporting structure, and further a reactive silicone compound having a radical polymerizable functional group and having a dimethylsiloxane structure as a repeating unit with a light energy through the use of an optical energy irradiation unit (see Japanese Patent Application Laid-Open (JP-A) No. 2005-115353).
However, it is known that the ability to retain a positive charge is significantly reduced on the surface layer of the above-noted photoconductor by repeatedly performing charging and exposing processes as compared to a typically used film in which a charge transporting material is dispersed in a binder resin. The cause is not yet clearly revealed, and the reason is assumed that the crosslinked film is deteriorated by some factors. When the photoconductor having the crosslinked film is positively charged by a transfer bias stated above, the positive charge is not retained on the photoconductor surface and is infused into the inside of the photosensitive layer. Most of the positive charge is trapped in mid of the photosensitive layer and when the photoconductor is next negatively charged, the positive charge is moved to the photoconductor surface to offset the negative charge generated on the photoconductor. As the result, the charge potential is reduced at the sites, and a residual negative image is generated after exposure of the photoconductor surface to thereby develop an abnormal image. It is known that the positive charge retention ability is reduced in proportion to the number of repeating times of charging and exposing. For deterioration of the crosslinked film, it can be considered that not only the outermost surface layer of the surface layer but also the inside of the surface layer are gradually deteriorated. When the inside of the surface layer is exposed outside, the inside of the crosslinked film has already been deteriorated, and residual negative images increasingly occur.